A non-aqueous heat transfer fluid is a heat transfer fluid formulated and used without any added water. ASTM International defines a non-aqueous coolant as “a glycol, diol, triol, or mixtures thereof, based heat transfer fluid containing less than 1.0% water when formulated and intended for final use without dilution with water.” In contrast, an aqueous, water-glycol heat transfer fluid is typically comprised of about 50 percent water, together with one or more polyhydric alcohol freezing point depressants.
Water in its liquid state has excellent heat transfer characteristics. Even when the water is combined with a polyhydric alcohol freezing point depressant, such as EG, the heat capacity and thermal conductivity of the resulting aqueous heat transfer fluid remain preferable for heat transfer applications as long as the fluid is maintained in its liquid state. The challenge with a water-glycol heat transfer fluid that contains a substantial amount of water is keeping it in its liquid state at all times, under the high heat density conditions of modern engines and their Exhaust Gas Recirculation (EGR) coolers. Typical water-glycol heat transfer fluids are operated close to their boiling points because their boiling points are dominated by the large percentage of water that they contain. The atmospheric boiling point of a solution of 50% EG and 50% water is 107° C. (225° F.), a temperature that is easily reached in the coolant passages of an engine. A typical engine cooling system is pressurized to raise the boiling point of the coolant. The pressure, at least partly, comes from the presence of water vapor from boiling of coolant. Water vapor does not transfer heat well, which can result in local hot spots. Non-aqueous heat transfer fluids have atmospheric boiling points that are far higher than the temperatures at which they are typically used. Localized boiling can still produce vapor but the vapor condenses immediately into colder surrounding liquid coolant, avoiding the accumulation and pocketing of vapor. Use of a high boiling point non-aqueous coolant, by preventing the accumulation of vapor, keeps liquid in contact with hot metal at all times, giving improved heat transfer, as compared to coolants that contain water under conditions when water vapor is present.
U.S. Pat. No. 8,394,287 describes the use of a heat transfer fluid prepared by blending non-aqueous EG, the glycol having the highest thermal conductivity and lowest viscosity, with propylene glycol (PG) to reduce the toxicity of the EG and to reduce its low temperature operating limit. PG, alone among glycols, does not supercool, and does not itself exhibit the usual symptoms of freezing (the formation of nodules or crystals), but rather simply gets thicker, until it will not pour at all at temperatures below about −60° C. PG is very viscous at low temperatures but was effective for lowering the LTOL of the EG to which it was added.
U.S. Patent Publication No. 2015/0284617 describes the use of PDO and or DEG, both of which supercool, as a means to reduce the LTOL of non-aqueous EG. The PDO and/or DEG combinations, despite the fact that they themselves supercool, are effective in reducing the LTOL of the EG, while also reducing the viscosity at low temperatures, as compared to EG with PG combinations.
The freezing point of a glycol that exhibits supercooling is a temperature well above the temperature where solidification related to low temperatures initiates. The supercooling temperature range of a glycol that exhibits supercooling is a freezing range; it begins to freeze at a lower temperature and remains frozen to a higher temperature. The published freezing point of a glycol that exhibits supercooling is actually the melting point of the solidified mass after it freezes. The published freezing point for neat EG is −12° C., a temperature well above the temperature that is required to be reached in order to initiate freezing, EG starts to freeze at −22° C. The LTOL of an anhydrous glycol that exhibits supercooling is a temperature just above the onset of freezing symptoms. If the LTOL is never reached, operation within the supercooling range is stable, without nodules, crystals or solidification. The LTOL for EG at −21° C. (9° C. colder than its −12° C. freezing point) can be easily breached if the EG is exposed to common wintertime weather in many parts of the world. Specifications currently under consideration by ASTM International require that a non-aqueous engine coolant have an LTOL of −40° C. or lower.
Researchers are dissuaded from studying small fractions of included water (e.g. percentages in the 5% to 10% range) with ethylene glycol as a means to reduce the LTOL of ethylene glycol or the viscosity of ethylene glycol because the accepted bodies of information show freezing points that are high in temperature for water percentages under 10 percent. None of the published freezing point temperatures for EG, with water percentages in the 5% to 10% range, are colder than −30° C.
It would be desirable to have heat transfer fluids that would 1) have boiling points much higher than traditional water-glycol coolants, 2) have LTOLs as good as non-aqueous coolants, and 3) have low temperature viscosities reduced on the order of 50 percent as compared to non-aqueous coolants.